It has become increasingly common in the silicon integrated circuit (IC) industry to design so-called System-on-Chip (SoC) solutions. The driving force behind this trend is the ever present desire to minimize the chip area required to perform the required functionality of the chip, and hence ultimately reduce the size and weight of the electronic devices into which the chip is incorporated.
Reducing the size of the components on the chip is not the only problem encountered in realising a more compact design. Another problem is that the components must be positioned closer to each other on the chip. This is particularly problematic for inductors. Inductors radiate magnetic fields that interfere with the operation of the circuitry surrounding them. The strength of the magnetic field is inversely proportional to the square of the radial distance from the inductor, thus the further away that the surrounding circuitry is located, the less interference it suffers as a result of the magnetic field. FIG. 1 illustrates a traditional spiral inductor 100 in which current enters the inductor at feed line 104, flows in an anticlockwise direction around each turn of the inductor as illustrated by arrow 102, and exits the inductor at feed line 106. This current generates a magnetic field in a direction perpendicular to the plane of the inductor. The strength of the magnetic field and its direction in the proximity of the inductor is illustrated in the plot of FIG. 2. The plot shows that the magnetic field strength has no dependence on the radial direction in the plane of the inductor. The magnetic field strength is dependent on the elevation and distance from the center of the inductor.
The magnetic field is particularly problematic for two inductors located on the same chip. Here, each inductor couples with the other inductor (called cross coupling). This is as a result of the current flowing through each inductor inducing a magnetic field which radiates to the other inductor and induces a current in that inductor. This cross coupling changes the operation of the inductor. For example, if an inductor is used as a voltage controlled oscillator (VCO) then the cross coupling changes the current through the inductor and hence changes the resonant frequency of the VCO. This coupling also constitutes an additional channel to pick up the noise.
It is not possible to prevent a magnetic field from being created by an on-chip inductor. However, efforts have been made to configure inductors so as to reduce the resultant magnetic field components experienced at distance from them. One configuration of an inductor that has been designed to address this problem is the so-called figure-of-8 inductor. Such an inductor 300 is shown schematically in FIG. 3. Current enters the inductor via a feed line 302 which joins the inductor at the centre of the inductor from a plane perpendicular to the plane of the inductor. The current flows around the structure in the directions indicated by the arrows. The current exits the inductor via a feed line 304 which exits the inductor in a plane perpendicular to the plane of the inductor. As a result of the crossover section in the middle of the figure-of-8 inductor, the current flows in a clockwise direction around the lower loop 306 of the inductor and in an anticlockwise direction around the upper loop 308 of the inductor.
As indicated using conventional notation on FIG. 3, the magnetic field created by the current flowing clockwise around the lower loop is directed into the page and the magnetic field created by the current flowing anticlockwise around the upper loop is directed out of the page. The field lines join such that most of the magnetic field components in the plane of the inductor are contained within the area of the figure-of-8 structure. A degree of cancellation of the magnetic field components is thus achieved at distance from the inductor in the plane of the inductor. Additionally, the magnetic field is cancelled in the plane of the inductor at distance along an axis that bisects the inductor structure such that the lower loop is on one side of the axis and the upper loop is on the other side of the axis.
This axis is marked 310 on FIG. 3. The cancellation is achieved because the upper loop and lower loop are the same size and shape and are perfectly symmetrical about the axis 310. The magnetic field components along the axis 310 radiated from the two loops are equal in magnitude but opposite in direction, and hence cancel leaving no resultant magnetic field.
The magnetic field in the proximity of the figure-of-8 inductor shown in FIG. 3 is considerably reduced compared to the magnetic field radiating from the spiral inductor shown in FIG. 1, and cancellation is achieved at distance along the axis 310. Thus, other components positioned along the axis 310 outside the boundary of the figure-of-8 inductor experience considerably less interference than they would if positioned in a corresponding place outside the boundary of the spiral inductor. However, interference/cross coupling is still a problem when using the figure-of-8 inductor because residual magnetic field components remain at distance from the figure-of-8 structure everywhere except along the axis 310 defined above.
Thus, there is a need for an improved inductor design which reduces the resultant magnetic field at distance from the inductor.